The adjustability of an antenna means in this description, that a resonance frequency or resonance frequencies of the antenna can be changed electrically. The aim is that the operation band of the antenna around a resonance frequency always covers the frequency range, which the function presumes at a given time. There are different grounds for the adjustability. As portable radio devices, like mobile terminals, are becoming smaller thickness-wise, too, the distance between the radiating plane and the ground plane of an internal planar antenna unavoidably becomes shorter. A drawback of the reducing of said distance is that the bandwidths of the antenna become smaller. Then, as a mobile terminal is designed to function in different radio systems having frequency ranges relatively close to each other, it becomes more difficult or impossible to cover frequency ranges used by more than one radio system. Such a system pair is for instance GSM1800 (Global System for Mobile telecommunications) and GSM1900. Correspondingly, securing the function that conforms to specifications in both transmitting and receiving bands of a single system can become more difficult. When the system uses sub-band division, it is advantageous if the resonance frequency of the antenna can be tuned inside sub-band being used at a given time, from the point of the radio connection quality.
A known way to adjust an antenna is the use of switches. For example a solution presented in FIG. 1 is known from the application publication FI 20021555. The basis of the solution is that a parasitic conductive element is connected to the ground by a switch. The antenna is a dual-band PIFA. The radiating plane 120 has a slot 125, which starts from an edge of the plane next to the short point S and ends at inner region of the plane. The slot 125 has such a shape that the radiating plane, viewed from the short point, is split into two branches. The first branch 121 skirts along edges of the plane and surrounds the second, shorter branch 122. The first branch together with the ground plane resonates on the lower operation band of the antenna and the second branch together with the ground plane in the upper operation band. The radiating plane 120 is a fairly rigid conductive plate, or metal sheet, being supported by a dielectric frame 180 to the radio device's circuit board 101 below the radiating plane. The conductive upper surface of the circuit board 101 functions as the ground plane 110 of the antenna and at the same time as the signal ground GND. The short-circuit conductor 111 and the feed conductor 112 are of spring contact type and the one and the same piece with the radiating plane.
A parasitic conductive strip 130 is in FIG. 1 attached or otherwise provided on a vertical outer surface of a dielectric frame 150, on that side of the antenna, where the feed conductor and the short-circuit conductor are located. The conductive strip 130 is in that case below the electrically outermost portion of the first branch 121, for which reason the connection of the conductive strip effects more strongly on the place of the antenna's lower operation band than on the place of the upper operation band. The switching arrangement is shown in FIG. 1 only by graphic symbols. The parasitic element 130 is connected to a switch SW, the second pole of which is connected to the signal ground through a component 150. The impedance of that component can be utilized, if desired displacements of operation bands can not be obtained merely by selecting the place of the parasitic element. The impedance is reactive, either purely inductive or purely capacitive; a resistive part is out of the question due to dissipations caused by it. In a special case the component 150 is a pure short circuit.
FIG. 2 shows an example of the effect of the parasitic element on antenna's operation bands in structures as described above. The operation bands appear from curves of the reflection coefficient S11 of the antenna. Curve 21 shows alteration of the reflection coefficient as a function of frequency, when the parasitic conductive strip is not connected to the ground, and curve 22 shows alteration of the reflection coefficient as a function of frequency, when the conductive strip is connected to the ground. When comparing the curves, it will be seen that the lower operation band is shifted downwards and the upper operation band upwards in the frequency axis. The frequency f1, or the centre frequency of the lower band for a start, is for instance 900 MHz and it's displacement Δf1 is for instance −20 MHz.
The frequency f2, or the centre frequency of the band for a start, is for instance 1.73 GHz and it's displacement Δf2 is for instance +70 MHz.
In the structures such as shown in FIG. 1, the adjusting of a multi-band antenna is obtained by means of additive components, which do not presume changes in the antenna's basic structure. The parasitic element is placed on a surface of a dielectric part, which is needed in the antenna structure in any case. However a flaw of that solution is, that there are only relatively limited possibilities to arrange both a proper impedance matching and a proper efficiency for the antenna. Moreover, if the influence of the use of the switch is desired to be limited only to certain operation band, keeping another operation band in its place can be difficult, in practice.
Instead of a discrete component, after the switch there can be a transmission line, implemented by the circuit board and being short circuited or open at the other end. The impedance of that kind of transmission line changes in a known way, when its length is changed. If the line's length is chosen just right, the antenna is provided with a desired displacement of an operation band. Using a multi-pole switch and several transmission lines, the operation band has corresponding number of alternative places. A transmission line in that kind of arrangement can be unpractically long so that it takes up remarkably the area of the circuit board.